1. Field of the Invention
The present invention relates to a drum brake. Specifically, the present invention relates to an improvement in realizing downsizing and reduction in power consumption of an electric motor used in an operating force producing mechanism in a drum brake, including the operating force producing mechanism for driving a power lever constituting an output portion of shoe operating force by the electric motor and a link mechanism for driving a shoe for controlling press force of the brake shoe exerted to a drum in accordance with brake force exerted to an anchor portion.
2. Description of the Related Art
Conventionally, there have been used various types of drum drakes for braking running of a vehicle and the drum brakes are classified into a leading trailing type, a two leading shoe type and a duo-servo type or the like in accordance with an arrangement of a brake shoe pressed to an inner peripheral face of a drum having substantially a cylindrical shape.
A drum brake of a duo-servo type is generally provided with a pair of brake shoes of a primary shoe and a secondary shoe arranged opposedly to each other in a drum having a cylindrical shape.
An inlet side of the primary shoe in a forward rotational direction of a drum is made to constitute on input portion and an outlet side thereof in the forward rotational direction of the drum is connected to an inlet side of the secondary shoe via an adjuster. Meanwhile, an outlet side of the secondary shoe is brought into contact with an anchor portion mounted on a backing plate and brake force (brake torque) operated to the primary shoe and the secondary shoe is received by the anchor portion.
Thereby, when the primary shoe and the secondary shoe are expanded and pressed to an inner peripheral face of the drum, the brake force operated to the primary shoe is inputted to the input side of the secondary shoe to operate the secondary shoe to press to the inner peripheral face of the drum and therefore, both of the primary shoe and the secondary shoe operate as leading shoes and brake force having very high gain can be achieved.
In comparison with a brake drum of a leading trailing type or a two leading shoe type, the above-described duo-servo type drum brake is provided with a number of advantages in which not only extremely high brake force can be achieved but also the brake drum is easy to downsize and integration of a parking brake is facilitated and so on.
However, such a duo-servo type drum brake is sensitive to a change in frictional coefficient of a lining of the brake shoe and, the brake force tends to be difficult to stabilize. Accordingly, a device for stabilizing the brake force is requested.
In view of such background, the applicant has already proposed a number of technologies for stabilizing brake force by controlling an output of a hydraulic type wheel cylinder operating to expand a pair of brake shoes in accordance with anchor reaction force.
Meanwhile, according to a brake apparatus of a vehicle in recent times, there is actively carried out intelligent formation of a brake function such as mounting of an anti-lock brake system or mounting of a traction control system. Further, in order to correspond to such intelligent formation, electric movement formation of a brake apparatus has been progressed.
Hence, the applicant has been carried out research and development of a drum brake for producing shoe operating force by a power lever driven by an electric motor and transmitting the shoe operating force produced by the power lever to respective brake shoe via a link mechanism.
The brake apparatus of such a constitution is suitable for the intelligent formation of the brake function by electric movement formation of a drive source. Further, when the link mechanism interposed between the power brake and the brake shoe is devised, the brake force can also be stabilized by restricting the shoe operating force produced by the power lever in accordance with the brake force, which is applicable to both of electric movement formation of the duo-servo type drum brake and stabilization of the brake force.
Meanwhile, in the case of the drum brake, the brake torque operated to the brake shoe in braking is varied by eccentricity of the drum or the like. The variation of torque caused by eccentricity of the drum or the like is a phenomenon occurring in an extremely short period of time and therefore, the rapid torque variation reacts to the link mechanism or the power lever supporting to press the brake shoe as impact force.
Hence, according to the conventional drum brake constituting the drive source by the electric motor, there is taken a countermeasure of giving allowance to allowable torque of the electric motor or promoting impact resistance of the electric motor such that the impact force derived from the torque variation does not constitute excessive load for the electric motor. However, such countermeasures constitute a factor for resulting in a large-size electric motor, and a brake apparatus becomes large and increases its weight.
Further, according to the conventional drum brake constituting the drive source by the electric motor, a power transmitting mechanism between the link mechanism for operating to expand the brake shoe and an output shaft of the electric motor is constructed by a reversible structure capable of transmitting power in two directions of a motor output transmitting direction for transmitting the output of the motor to the brake shoe and a brake torque transmitting direction for transmitting the torque operating to the brake shoe to the side of the motor in braking. Therefore, there also poses a problem that when constant brake force is maintained in, for example, stopping a vehicle or the like, electricity conduction is obliged to continue in the electric motor, power consumption of the electric motor is increased and load to a battery or a generator mounted to the vehicle is increased.
The invention has been carried out in view of the above-described situation. Accordingly, it is an object of the present invention to provide a drum brake capable of realizing downsizing of an electric motor so that impact resistance of the electric motor caused by torque variation in braking can be promoted without achieving an increase in an allowable torque and promotion of impact resistance of the electric motor per se. Further, it is an object of the present invention to provide a drum brake capable of achieving a reduction in power consumption of the electric motor so that a drum brake capable of maintaining brake force even when electricity conduction to the electric motor is stopped in braking. Specifically, the reduction in power consumption can be achieved so that a power transmitting mechanism between the electric motor and a link mechanism for operating to expand a brake shoe is made irreversible by which although an output of the motor is transmitted to the link mechanism, torque transmission from the link mechanism to the side of the electric motor is restricted.
In order to achieve the above-described object, according to the present invention, there is provided a drum brake including a pair of brake shoes opposedly arranged inside a drum; an operating force producing mechanism for driving a power lever constituting an output portion of a shoe operating force by an electric motor; a link mechanism for driving the shoes having an input lever for receiving the shoe operating force outputted from the power lever, wherein the link mechanism presses the pair of brake shoes to a drum and operates a brake restricting force to the input lever in a direction of a reducing operation of the shoe operating force in accordance with a brake force operated from the brake shoes to an anchor portion for controlling a press force of the respective brake shoes to the drum. The drum brake further includes a damper spring between the power lever and the input lever. The damper spring transmits the shoe operating force outputted from the power lever to the input lever and absorbs an impact force transmitted from the brake shoes to the power lever via the link mechanism in braking.
According to the above-described constitution, when the impact force is transmitted from a side of the brake shoes to the operating force producing mechanism via the link mechanism by rapid torque variation caused by eccentricity of the drum or the like in braking, the impact force is absorbed by elastically deforming the damper spring arranged between the input lever of the link mechanism and the power lever.
Therefore, the impact force is absorbed between the input lever and the power lever and is not transmitted to the electric motor of the operating force producing mechanism via the power lever.
Further, preferably, in the drum brake, plural pieces of the damper springs may be arranged in parallel between the power lever and the input lever.
When constituted in this way, assuming, for example, compression coil springs as the damper spring, in comparison with a case of constituting the damper spring by a single spring, a large spring constant can be ensured by small dimensions by an amount of reducing a diameter of winding the individual coil spring. Further, there is also promoted a degree of freedom of arranging parts such as distributing and arranging, for example, the respective compression coil springs at vacant spaces on both sides of the power lever.
Further, preferably, in the drum brake, the damper spring may be set with a spring constant such that a change in a deceleration by the impact force transmitted from the brake shoes to the power lever via the link mechanism in braking becomes equal to or smaller than 0.05 G.
When the change in the deceleration is restrained to the degree that kickback or the like accompanied by the change in the deceleration becomes very weak to a degree which cannot be felt by a driver or the like.
Further preferably, in the operating force is producing mechanism of the drum brake, a power transmitting system for transmitting an output of the electric motor to the power lever is constituted by integrating a worm wheel apparatus for preventing power from being transmitted from a side of the power lever to a side of the electric motor.
When constituted in this way, the power transmission mechanism between the link mechanism and the electric motor for operating to expand the brake shoes is made irreversible by which although the output of the motor is transmitted to the link mechanism, the torque is restricted from being transmitted from the link mechanism to the side of the electric motor. Accordingly, even when electricity conduction to the electric motor is stopped in braking, the braking force can be maintained.